1. Field of the Invention
The present invention relates to an image forming apparatus able to be used in a printer, a copier, or a combination of them, and more particularly, to an image forming apparatus in which a target color image is formed by transferring and superposing monochromatic color images on photo conductors to a transfer belt sequentially, and a moving speed of the transfer belt is controlled to be constant so as to suppress color deviation.
2. Description of the Related Art
Among electrophotographic image forming apparatuses, market demand for color image forming apparatuses, such as color printers, color copiers, is rapidly increasing, and especially recently, users are requiring a speed of color image formation comparable to that of monochromatic image formation.
In order to meet this demand, a tandem engine configuration is more and more adopted in the color image forming apparatus. In a color image forming apparatus of the tandem engine configuration, a set of a photoconductor, a developing device, a writing optical system, and other devices, is provided for each color component of a color image to be formed by the color image forming apparatus (referred to as “target color image” below), and monochromatic toner images corresponding to the respective color components of the target color image are formed on the respective photoconductors, and these monochromatic toner images of different colors are then sequentially transferred to a recording sheet, thereby resulting in a full color image on the recording sheet.
There are two types of the color image forming apparatuses of the tandem engine configuration; one involves “direct transfer”, and the other one involves “indirect transfer”.
FIG. 1 is a schematic view showing a configuration of a direct-transfer image forming apparatus 10a. 
In the direct-transfer image forming apparatus 10a, a transfer belt 151, which is a flexible belt, is wound on a driving roller 152 (driven by a not-illustrated motor) and a driven roller 153. The upper part of the transfer belt 151 is placed between photo conductor drums 102C, 102M, 102Y, 102K for forming cyan (C), magenta (M), yellow (Y), and black (K) monochromatic images, respectively, and the correspondingly arranged transfer rollers 104C, 104M, 104Y, 104K, and is driven to move in the left direction in FIG. 1.
The surface of each of the photo conductor drums 102C, 102M, 102Y, 102K is uniformly charged by a not-illustrated charging device. Optical writing units 101C, 101M, 101Y, 101K, which are controlled by a controller 110, emit modulated laser beams according to the cyan, magenta, yellow, and black monochromatic image data onto the charged surfaces of the photo conductor drums 102C, 102M, 102Y, 102K. Thereby, the charged surfaces of the photo conductor drums 102C, 102M, 102Y, 102K are neutralized, and latent images are formed on the surfaces of the photo conductor drums 102C, 102M, 102Y, 102K.
When the thus formed latent images move between the corresponding photo conductor drums 102C, 102M, 102Y, 102K and the developing devices 103C, 103M, 103Y, 103K, cyan, magenta, yellow, and black monochromatic toners stored in the respective developing devices 103C, 103M, 103Y, 103K are added onto the respective latent images by the developing devices 103C, 103M, 103Y, 103K, and thereby, the latent images are converted into visible toner images.
On the other hand, a recording sheet is conveyed by a pair of conveyance rollers 106 and is closely attached onto the upper part of the transfer belt 151 to move together with the transfer belt 151. When the recording sheet moves through pairs of the photo conductor drums 102C, 102M, 102Y, 102K and the transfer rollers 104C, 104M, 104Y, 104K, sequentially, the cyan, magenta, yellow, and black monochromatic toner images on the photo conductor drums 102C, 102M, 102Y, 102K are sequentially and directly transferred onto the recording sheet, and are superposed on the recording sheet. As a result, a superposed full color image is formed on the surface of the recording sheet.
The recording sheet carrying the superposed full color image is further conveyed into a fusing unit 107. When the recording sheet passes through the fusing unit 107, the recording sheet is heated and pressed, and thereby the superposed full color image is fused and fixed on the recording sheet.
FIG. 2 is a schematic view showing a configuration of an indirect-transfer image forming apparatus 10b. In FIG. 2, same reference numerals are used for the same elements as those in FIG. 1.
In the indirect-transfer image forming apparatus 10b, a transfer belt 151 is wound on a driving roller 152, which is driven by a not-illustrated motor, and a driven roller 153. The lower part of the transfer belt 151 is disposed between photo conductor drums 102C, 102M, 102Y, 102K for forming cyan (C), magenta (M), yellow (Y), and black (K) monochromatic images, respectively, and the correspondingly arranged transfer rollers 104C, 104M, 104Y, 104K, and is driven to move in the left direction in FIG. 2.
The surface of each of the photo conductor drums 102C, 102M, 102Y, 102K is uniformly charged by a not-illustrated charging device. Optical writing units 101C, 101M, 101Y, 101K, which are controlled by a controller 110, emit modulated laser beams according to the cyan, magenta, yellow, and black monochromatic image data onto the charged surfaces of the photo conductor drums 102C, 102M, 102Y, 102K. Thereby, the charged surfaces of the photo conductor drums 102C, 102M, 102Y, 102K are neutralized, and latent images are formed on the surfaces of the photo conductor drums 102C, 102M, 102Y, 102K.
When the thus formed latent images move between the corresponding photo conductor drums 102C, 102M, 102Y, 102K and corresponding developing devices 103C, 103M, 103Y, 103K, cyan, magenta, yellow, and black monochromatic toners, which are stored in the respective developing devices 103C, 103M, 103Y, 103K, are added onto the respective latent images by the developing devices 103C, 103M, 103Y, 103K, and thereby, the monochromatic latent images are converted into visible monochromatic toner images.
The cyan, magenta, yellow, and black monochromatic toner images on the photo conductor drums 102C, 102M, 102Y, 102K are then sequentially transferred onto a portion of the transfer belt 151 when the portion sequentially passes through each pair of the photo conductor drums 102C, 102M, 102Y, 102K and the transfer rollers 104C, 104M, 104Y, 104K, and then superposed on the portion of the transfer belt 151, resulting in a full color toner image on the transfer belt 151. The full color toner image is conveyed while the transfer belt 151 is moving in the left direction.
On the other hand, a recording sheet is conveyed at an appropriate timing by a pair of conveyance rollers 108 to the position between the driving roller 152 and a secondary transfer roller 160. The recording sheet is moved between the driving roller 152 and the secondary transfer roller 160, while being firmly held by the driving roller 152 and the secondary transfer roller 160. The full color toner image is transferred (the second transfer), onto the recording sheet when the recording sheet passes between the driving roller 152 and the secondary transfer roller 160 at an appropriate timing.
The recording sheet carrying the full color toner image is conveyed further into a fusing unit 107. When the recording sheet passes through the fusing unit 107, the recording sheet is heated and pressed, and thereby the full color image is fused and fixed on the recording sheet.
In either the direct-transfer image forming apparatus 10a or the indirect-transfer image forming apparatus 10b, the toner images of different colors on the respective photo conductor drums 102C, 102M, 102Y, 102K, which are at different positions on the transfer belt 151, are transferred directly to the recording sheet, or to the transfer belt 151.
If the images of different colors are formed on the respective photo conductor drums 102C, 102M, 102Y, 102K and transferred to the transfer belt 151 at the same time, it is apparent that the toner images of different colors are transferred to different positions on the transfer belt 151, that is, color deviation occurs. To avoid this problem, the controller 110 adjusts the timings of outputting monochromatic image data signals to the respective optical writing units 101C, 101M, 101Y, 101K by incorporating time delays corresponding to intervals of the photo conductor drums 102C, 102M, 102Y, 102K along the transfer belt 151.
For example, if the intervals of the photo conductor drums 102C, 102M, 102Y, 102K along the transfer belt 151 are 10.0 cm, and the moving speed of the transfer belt 151 is 10.0 cm/second, the timings of writing the monochromatic images by the respective optical writing units 101C, 101M, 101Y, 101K to the respective photo conductor drums 102C, 102M, 102Y, 102K are shifted by one second consecutively.
However, even though the driving roller 152 drives the transfer belt 151 at a constant rotational speed, if the moving speed of the transfer belt 151 is not constant within one cycle, that is, the distance through which the transfer belt 151 moves per unit time, for example, per second, is not a constant, then transfer positions, at which images of different colors are transferred, are different. As a result, color deviation occurs in the superposed color image transferred on the recording sheet or the transfer belt 151.
For this reason, in order to suppress color deviation in the color image forming apparatus having the tandem engine configuration, it is required that the moving speed of the transfer belt 151 be constant within one cycle, or at least within the period when the toner image is being transferred.
The moving speed V of the outer surface of the transfer belt 151, to which the toner image is transferred, can be expressed as below,V=(R+r)ωt  (1)
where, R represents the radius of the driving roller 152, r represents the thickness of the transfer belt 151, and ω represents the angular speed of the driving roller 152.
It is relatively easy to machine the radius R of the driving roller 152 at high precision. However, because the transfer belt 151 is film-like, that is, it is relatively long, and relatively thin and narrow, it is difficult to fabricate the transfer belt 151 to have a uniform thickness, especially in the longitudinal direction (that is, the rotation direction).
If the thickness of the transfer belt 151 is not uniform over the total length thereof, even if the speed of the driving roller 152, which drives the transfer belt 151, is controlled to be constant, the moving speed of the transfer belt 151 ends up varying periodically.
Specifically, if the lower limit of the thickness of the transfer belt 151 is r_min, the upper limit of the thickness of the transfer belt 151 is r_max, variation Δr of the thickness of the transfer belt 151 over the length thereof is in the following range:0≦Δr≦(r_max−r_min).
With Δr, the equation (1) can be rewritten asV=(R+(r_min +Δr)ωt  (2)
Δr is a function of a position on the transfer belt 151 along the longitudinal direction. Below, a certain position on the transfer belt 151 along the longitudinal direction is represented by x, and Δr is expressed as Δr(x) . Because Δr(x) changes with the contacting position of the outer circumference of the driving roller 152 with the transfer belt 151, even if the angular speed of the driving roller 152 is constant, the moving speed V of the outer surface of the transfer belt 151 changes.
As described above, in the color image forming apparatuses having the tandem engine configuration, variation of the thickness of the transfer belt 151 causes transfer positions, at which images of different colors are transferred, to be different from each other, and as a result, color deviation occurs in the output color image.
In principle, it is possible to prevent the color deviation by managing to control the moving speed of the transfer belt 151 to be constant regardless of thickness variation of the transfer belt 151 in the longitudinal direction. Specifically, the angular speed of the driving roller 152 is controlled to be in such a way that the angular speed of the driving roller 152 is decreased when the portion of the transfer belt 151 contacting the outer circumference of the driving roller 152 is thick, and is increased when the portion of the transfer belt 151 contacting the outer circumference of the driving roller 152 is thin.
Followings are the methods proposed so far for suppressing the variation of the moving speed of the transfer belt 151.
In the method disclosed in Japanese Laid-Open Patent Application 6-127037, a striped pattern is formed on the transfer belt, and the moving speed of the transfer belt is measured by detecting the pattern using a sensor, and a polygonal motor is controlled by feeding back the measured speed.
In the method disclosed in Japanese Laid-Open Patent Application 11-174932, an encoder is attached to the axle of the driven roller that supports the transfer belt, and the speed of the transfer belt is controlled by feeding back the output from the encoder.
In the method disclosed in Japanese Laid-Open Patent Application 2001-51479, variation of the thickness of the transfer belt is measured in advance, and based on the measured variation of the belt thickness, the write timing of different colors are controlled, thereby reducing variation of the moving speed of the transfer belt.
However, in the method disclosed in Japanese Laid-Open Patent Application 6-127037, although it is possible to measure the moving speed of the outer surface of the transfer belt accurately, it is difficult to form the striped pattern on the belt, and this increases the fabrication cost.
Further, depending on the control method used in the control, a sensor of a high resolution may be necessary, and a circuit exclusively used for the feedback control of the signals from the sensor becomes necessary. Consequently, the apparatus becomes quite expensive.
In the method disclosed in Japanese Laid-Open Patent Application 11-174932, because additional parts like the driven roller, the encoder, and a circuit exclusively used for the feedback control are necessary, the apparatus becomes quite expensive.
In the method disclosed in Japanese Laid-Open Patent Application 2001-51479, because the thickness variation of the transfer belt is measured in advance in factories before shipment, and the write timing of different colors are controlled based on the measured data, it is not necessary to install detection devices, such as encoders, in the apparatus, and thereby, cost of the system is relatively low.
However, variations in the thickness of a transfer belt are caused by fluctuations in the fabrication condition of the transfer belt, and different transfer belts have respectively different thickness uncertainties. Therefore, measured thickness variation of one belt cannot be used for other belts. For this reason, when exchanging a transfer belt, it is necessary to set data of thickness variation of the belt to be used into the apparatus.
Because the transfer belt is a consumable article, after printing a certain number of recording sheets, the transfer belt has to be exchanged. Among the color image forming apparatuses, usually it is the user himself that exchanges the belt unit of a color printer, while usually a service personnel performs maintenance on a facsimile machine or a copier for the users. When the user exchanges the belt unit, it becomes necessary to attach belt thickness variation data with the new belt, and the user has to set the data into the printer by operating an operational panel. This operation is cumbersome, and if the setting is wrong by mistake, expected printing quality cannot be obtained, or the printing quality may be much degraded.